Chemical and radiochemical characterization of phosphogypsum from Poland

Over 75% of the global phosphate fertilizers is produced using phosphoric acid as an intermediate, which is produced via the wet-phosphoric acid process, generating 4–5 tons of the byproduct phosphogypsum per ton of the phosphoric acid Eq. (1) [1]. The amount of phosphogypsum produced worldwide is estimated at about 100–280 million tons per year. In Poland alone, 2–3 million tons of phosphogypsum are produced annually [2]. (1) [ Ca3(PO4)2 ]3CaF2+10H2SO4+20H2O→6H3PO4+10(CaSO4∗2H2O)+2HF \[\begin{matrix} {{\left[ \text{C}{{\text{a}}_{3}}{{\left( \text{P}{{\text{O}}_{4}} \right)}_{2}} \right]}_{3}}\text{Ca}{{\text{F}}_{2}}+10{{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}+20{{\text{H}}_{2}}\text{O} \\ \to 6{{\text{H}}_{3}}\text{P}{{\text{O}}_{4}}+10\left( \text{CaS}{{\text{O}}_{\text{4}}}*2{{\text{H}}_{2}}\text{O} \right)+2\text{HF} \\ \end{matrix}\]

Phosphogypsum contains approximately 96% of calcium sulfate dihydrate (CaSO₄ × 2H₂O) [3, 4], various impurities such as phosphates, sulfates, fluorides, heavy metals, and trace elements such as lead, arsenic, cadmium, zinc, chromium, copper, and antimony [5]. The impurities content varies widely, influenced by phosphate rock origin and production-process factors. Phosphate rocks also contain radioactive elements (mainly uranium and thorium), which partially remain in the phosphogypsum as a result of the “wet process”. The purification is challenging and expensive due to the chemical similarities of radium, often occurring in this material, with calcium [6]. The concentration of the radionuclides present usually exceeds that of the natural background. As a result, phosphogypsum can be classified as a technologically enhanced naturally occurring radioactive material (TENORM) [3].

Phosphogypsum, typically stored in expansive open heaps, poses environmental threats such as water, land, and atmospheric pollution. Nevertheless, with proper management, it does not need to be classified as waste. Ongoing efforts aim to repurpose phosphogypsum for applications such as construction materials, agricultural soil amendments, and industrial processes, minimizing its environmental impact [4, 7]. Phosphogypsum can possess the potential to be an important source of rare earth elements (REEs) and be a viable alternative of flue gas desulfurization (FGD) gypsum [8]. The radioactivity of phosphogypsum is usually relatively low, and it meets most national regulations to be used as a building material [4].

To comprehensively grasp the radiological and chemical attributes of phosphogypsum, a diverse array of analytical methods is essential, e.g., gamma and alpha spectrometry techniques for radioisotopes and mass spectrometry for other elements. Polish phosphogypsum from the former Chemical Wizów Plant is notable for its low radioactivity, because it predominantly originates from the processing of Kola Peninsula apatite [9, 10]. Apatite belongs to igneous phosphates, which are characterized not only by a very low content of radioactive elements, but also by a higher concentration of REEs (approximately 1%) than sedimentary phosphate rocks.

The main objective of this work is the examination of the chemical and radiochemical characterization of Wizów phosphogypsum stacks and the assessment of their potential as an important source of REEs and a viable alternative of gypsum. The consortium of Institute of Nuclear Chemistry and Technology with partners from Europe and Morocco is actively implementing the “Phosphogypsum Processing to Critical Raw Materials” project under ERA-MIN3. This project aims to develop innovative technologies for processing phosphogypsum stacks, leveraging its richness in REEs to meet the growing demand in the European high-tech industry, while simultaneously repurposing the gypsum matrix for cost-effective construction materials, aligning with EU’s critical raw materials list [11]. The stacks of the former Wizów Chemical Plant contain 5–12 million tons of phosphogypsum from the processing of apatite from the Kola Peninsula. It is suspected that there are 6000–10 000 tons of REEs in the Wizów phosphogypsum stacks.

Ten representative samples of phosphogypsum were collected from various locations within all the three Wizów’s stacks (Figs. 1a and 1b). Phosphogypsum is characterized by its off-white color, dampness, and a powdery or granular texture as shown in Figs. 1c and 1d. Additionally, it contains 50–75% of particles finer than 0.08 mm and maintains a moisture content of approximately 11%.

Fig. 1.

The studied samples were grounded to a grain size ≤200 μm, homogenized thereafter, oven dried at 65°C for 5 h, and analyzed as shown in Fig. 2.

Fig. 2.

Inductively coupled plasma mass spectrometry (ICP-MS) measurements were performed using the Perkin-Elmer ELAN DRC II mass spectrometer with specific components and operational settings, including radio frequency power, lens voltage, dual detector mode, and argon flow rates. An indium internal standard was applied at a concentration of 5 ng/ml.

Prior to the ICP-MS analysis, mineralization is necessary (procedure: step 1: 120 mg of sample, 6 ml conc. HNO₃ + 2 ml conc. HF, 240°C, time 2 h; step 2: excess of fluoride complexing with 12 ml of 4% H₃BO₃, 200°C, time 80 min).

High purity germanium (HPGe) radiation detectors were used to measure the radioactivity in the samples (for the radionuclides determined after two months of equilibrium establishment). Efficiency was calibrated with standard sources and the background was subtracted. The method ensured accuracy and minimized self-absorption effects. The concentrations of ²³⁵U, ²²⁶Ra, ²³²Th, and ⁴⁰K in the samples were determined using a slightly modified standard method in sample-preparation techniques and data analysis. The EFFTRAN algorithms for enhanced background subtraction and peak identification in the spectral analysis were used. The results were referred to a certified reference material (IAEA-443). Samples ranged from 800 g to 850 g, with a measurement time of 250 000 s. The measurements were repeated two times to confirm the correctness of the obtained results.

The combined analysis of Wizów’s phosphogypsum using ICP-MS and gamma spectrometry has yielded valuable insights. ICP-MS revealed diverse elemental concentrations, including significant cerium and lanthanum levels (2036–2234 mg/kg and 1408–1545 mg/kg, respectively), indicative of REEs. Iron, with concentrations ranging from 1024 mg/kg to 1124 mg/kg, also played a substantial role in the sample’s composition.

Gamma spectrometry analysis of Wizów’s phosphogypsum highlights the coexistence and impact of the uranium and thorium decay series. Radionuclide concentrations are low and do not exceed three times the background radioactivity. These parameters make Wizów’s phosphogypsum a promising substitute for gypsum, given its significant REE metal concentrations and low diversity in radionuclide concentrations across the three stacks, indicating its potential as a valuable REE reserve. The demand for REEs is continuously growing due to their high requirement in many applications. The studies will be continued in the frame of the ERAMIN project.